PDA

View Full Version here: : Pier pressure

Jeffkop
05-02-2012, 10:26 AM
Hello again

Well after a patient 2 year wait Im finally starting my observatory construction. I have gone the way of a 2.3m Sirius and now need to consider the pier first. Im finding the more I delve into piers the more questions I need answers to ... Sheeeesh.

Anyway its occured to me that from a financial perspective a concete pier is a hands down winner ... just a tube, some reo, a stick to work the cement and a sloppy mix. Concerns about observatory relocation aside ... are there any astro-photography related issues that preclude this type of pier .. or rather ... are there any significant benefits of steel over concrete ... and what is the suggested diameter of a concrete pier that will most likely be 1800mm out of the ground.

h0ughy
05-02-2012, 10:39 AM
have a look at the thread on the doghouse observatory, mark4darkskies observatory - he has an excellent record and plans

sheeny
05-02-2012, 11:24 AM
I did some calcs a while ago comparing steel and concrete piers. They are here on IIS somewhere - I'll try to find the thread. Basically for any given diameter, a steel pier is 10x stiffer than a concrete one. I had to make some assumptions about the grade of concrete used as that affects the young's modulus of concrete, but regardless no concrete mixture comes close to steel.

Now that said, remember the stiffness of a columm or beam in bending is proportional to the fourth power of the diameter. The fourth root of 10 is 1.78, so a 178mm diameter concrete pier is as stiff as a 100mm steel one. But... there's another part to the equation. The amplitude of vibration for any given energy (size of bump/knock) is inversely proportional to the natural frequency of the pier. The natural frequency increases with stiffness and decreases with mass. So a concrete pier of equivalent stiffness to a steel one, will have a lower natural frequency and therefore a higher amplitude of vibration if you bump it.

Don't get too hung up about it though. Unless you are in the habit of tapping your pier in time with the music, you don't need a super stiff pier once you're out of the wind (in your obs!), as long as its strong enough (minimum 100mm steel / 178mm dia concrete IMO at <1000 tall - I'd say at least 150NB steel @ 1800 tall).

The most important thing to worry about IMO is the size of the top of the pier so you have maximum / good scope clearance in all positions.

Al.

sheeny
05-02-2012, 11:43 AM
Here's my post:

http://www.iceinspace.com.au/forum/showpost.php?p=388551&postcount=30

Al.

Jeffkop
05-02-2012, 12:07 PM
Thanks David thank Al ... yes I have always been conscious of the diam of the top of the pier as regards clearance especially with camera attached ... I have a Titan I bought 2 years ago to go on it with a MAL adapter that is 180mm in diameter ... So based on those calculations Al a concete pier of 197mm would be adequate. (for 1800mm) ... Im thinking if I do it 250mm and have a small reduction at the top ( say over the last 30mm or so) down to 180mm for the Titan adapter then this might just do it.

koputai
05-02-2012, 06:44 PM
With no calculations involved, a pier of 197mm diameter and 1800mm high sounds a bit too thin for the height.

ChrisM
05-02-2012, 11:27 PM
Jeff, I made my concrete pier 450 mm dia and it's 2.2 m high. I've never detected any low-frequency vibration after the pier is struck, so I reckon that it's about the right dia/height. I agree that a 200 mm pier 1800 mm high made from concrete would seem too thin.

To avoid the potential for the pier interfering with equipment on top, and to provide a means of changing the equipment height at some time in the future, I opted for a steel pier on top of the concrete pier.

See http://www.iceinspace.com.au/forum/showthread.php?t=36222 for a pier pic (I think at around post #7).

Chris

Jeffkop
06-02-2012, 09:01 AM
Thanks for these replies ... Confusion here ... but perhaps I still have a direction to head ... In my reply to Al I suggested that I was thinking of a 250mm size with a reduction down to 180mm over the top 30mm or so. It looks as though the 250mm is still too conservative however.

My suggested 1800mm high is probably a little over estimated too. Its based on this:

If the wall height of the observatory is 1500mm I would imagine I would have to have the scope sitting to at least 500mm of that.

So that equates to 1000mm in the observatory and I reckon it will be 400-500mm to the ground ... so the overall length of the pier somewhere around 1400-1500mm

Jeff

Terry B
06-02-2012, 09:37 AM
I think that there is to much hype about how big the pier needs to be. Mine is cast iron about 150mm diameter. If I hit it it will move but who hits their pier?
The mount should be balanced on the top of it pretty well so there is very little lateral force on it except perhaps when you are slewing. This isn't when you are imaging so it doesn't matter.

The mount should balance on the pier with no connecting bolts.

jenchris
06-02-2012, 12:45 PM
Mine is 230mm x 1.7 and it doesn't have any LF vibes -
It is Mains HP water pipe full of concrete and steel.
I tend not to clout my pier whilst imaging and it seems to stay quite still -
mind you I only built the Obsy last week and I've only had 2 hours to set it up last night and the night before.
The moon didn't do much more than allow me a chance to drift align.

Jeffkop
06-02-2012, 04:22 PM
Thanks for your posts ... yes I think the pier design could easily be way over engineered .. but not according to the owner of astro engineering .. he see's it as another machine in the imaging process and I am not disagreeing with that .. but realise there is a price to pay for his standards and like you both say ... who is going to tap their mount when imaging anyway. There are all different types of piers in use out there and a lot of happy owners to got with them so I dont see it as a very daunting exercise to count myself amongst those in the end. I will be carrying some weight on the mount ... A titan and BRC250 with a Sky90 guidescope plus cameras and counter weights etc is going to add up to some Kg's but like Terry says, if the pier is vertical and the system balanced then Im going to be in a good spot.
I have a great anticipation .. Ive been waiting for 2 years with all this equipment ready to go into an observatory.

jenchris
06-02-2012, 04:35 PM
The weight you have on top makes a difference only when you move it.
As you say, it is balanced.
If it was hanging out sideways, it would be a difficult thing to restrain but it's not.

Jeffkop
08-02-2012, 10:33 AM
Update everyone ... am going with a steel pier approx 1000mm high. 180mm diam pipe 6mm thick welded to a 380mm diam 10mm thick base plate ... 4 x 6mm webbings ... 180mm top plate 10mm thick with 4 threaded holes for Losmandy MAL adapter.

Will raise the telescope footing out of the ground via 400mm diam formatube (as mark4darkskies approach http://www.pbase.com/gailmarc/image/90025641) ... approx 500mm high

Any opinions welcome

Thank you

koputai
08-02-2012, 11:03 AM
That sounds pretty good Jeff. The wider apart you can get the bolt that hold the steel flange to the concrete base the better. Six or eight M12 (or M16) bolts on a 340mm PCD would be the go.

Cheers,
Jason.

Marke
08-02-2012, 12:56 PM
Hmm think mines going to be way over kill

koputai
08-02-2012, 02:52 PM
As the actress said to the Bishop, there's no such thing as too stiff.

Cheers,
Jason.

pmrid
08-02-2012, 04:39 PM
Jeff, are you sure about the pier being 1800 out of the ground? With a decent mount on top of it, your scope will be 2 metres or more off the ground.

Peter

Jeffkop
08-02-2012, 04:55 PM
Thanks Pete .. please see revised pier plans